The Controller of Hippos and Yorkies

How does an organ know when to stop growing? It may sound like a riddle, but it’s a serious biological question with the potential for grave consequences. During development, an organism grows from a single cell up to trillions of cells. If that growth process overshoots its goal and doesn’t stop generating new cells, the result can be the unrestrained proliferation of cancer. Scientists have thus looked for the regulators of that growth, a search that led them to a cast of unusual characters: hippos, Yorkies, and warts.

That colorful menagerie is the result of research in fruit flies, where naming conventions steer away from the cold acronyms used by the rest of biology. Researchers of the fruit fly Drosophila melanogaster run screens where individual genes are deleted or suppressed, then name the gene according to the unusual appearance or activity this modified fly displays. So when a genetic deletion created a fly with organs of unusually large size, researchers named that missing gene Hippo. Conversely, the name Yorkie was assigned to a gene that, when deleted, produced a fly that grew abnormally small organs.

In the early 2000s, researchers determined that Hippo and Yorkie – and a handful of other genes found to control organ size – were all part of the same system, dubbed the Hippo-Salvador-Warts (HSW) signaling pathway. These elements were not exclusive to flies, but found in a host of other organisms, suggesting that the system goes far back in evolutionary time as a critical controller of cell function. Early returns also indicate that the HSW pathway is a likely contributor to human cancers, said Rick Fehon, professor and chair of molecular genetics and cell biology at the University of Chicago.

“The basic components are in yeast, worms, flies, and humans, so it’s a really fundamentally conserved pathway,” Fehon said. “It’s a pretty fresh field in general, and I think the mammalian cancer implications are far from having been fully explored.”

While the Hippo to Salvador to Warts to Yorkie pathway has been firmly established, scientists are still looking for how elements upstream turn the pathway on and off. In a new paper published this week in the journal Developmental Cell, Julian Boggiano and Pamela Vanderzalm of Fehon’s laboratory discovered one of these HSW pathway “switches,” and lengthened the cellular chain of how organ size is regulated.

Boggiano and Vanderzalm were looking for proteins that interact with another cell growth regulator called Merlin, a gene responsible for the disease neurofibromatosis in humans. One by one, they depleted a family of proteins called the Sterile 20 kinases, looking for an element that regulates Merlin activity. In the process, they found that suppressing one gene, called Tao-1 (this name originates from studies in mammals, not flies), created a fly that looked similar to Hippo, displaying an abnormal growth of organs called imaginal discs that form the wings and eyes of adult flies (seen above).

“We were looking for one thing, and serendipitously found something else,” said Vanderzalm, a postdoctoral fellow. “Imaginal discs undergo about 1,000 fold growth in four days. During that time they go from about 50 cells to 50,000 cells. You can tell right away that the overall shape is disrupted and wherever we’ve driven Tao-1 RNAi, those cells have a growth advantage, and they’ve overgrown relative to the remaining wild type cells in that tissue. They’re dividing more frequently.”

“That was when we realized it was probably a new component of this pathway,” said Boggiano, a graduate student in the Committee on Development, Regeneration, and Stem Cell Biology.

Because suppressing Tao-1 produced a similar effect as deleting Hippo, the researchers hypothesized that Tao-1 had a previously unrecognized influence upon the HSW pathway. Subsequent experiments tested whether Tao-1 was capable of activating Hippo in a test tube – which it did – and depleting Tao-1 in an organism led to suppressed activity of the HSW pathway. The evidence added a new component to the HSW flow chart, placing Tao-1 directly upstream of the Hippo-Salvador-Warts-Yorkie chain of events and giving researchers another potential candidate for cancer-causing mutations.

“This gives you another part of the pathway, and in terms of clinical relevance, it gives you something else to look at in diseases that might involve this pathway and possibly gives you a target to manipulate,” Fehon said. “It gives you another link in the chain, and it gives you a better understanding of how Hippo itself is activating this pathway. It’s still mysterious but it’s less mysterious than it was. We now know one clear mechanism by which it can be activated.”

If the clinical potential of Tao-1 and the HSW pathway pans out, it will only be the latest medical advance to originate in the humble fruit fly. Previous genes found in Drosophila (with equally colorful names such as Hedgehog and Wingless) were later demonstrated to be relevant for the construction of the human brain, the regulation of stem cells, and the formation of many different cancers. As genetic screens grow more and more comprehensive in the flies, the chance of finding an entirely new important cellular pathway may be dwindling, Fehon said, leaving the HSW pathway as a potential last hurrah.

“It’s such a new pathway that we’ve only barely touched the surface of the medical relevance,” Fehon said. “If you look at the past history of signaling pathways in flies, they’ve all proven to be incredibly medically relevant. These were all started in flies, and this is the next frontier”